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Monday, October 31, 2016

Modified Gravity vs Particle Dark Matter. The Plot Thickens.

They sit in caves, deep underground. Surrounded by lead, protected from noise, shielded from the warmth of the Sun, they wait. They wait for weakly interacting massive particles, short WIMPs, the elusive stuff that many physicists believe makes up 80% of the matter in the universe. They have been waiting for 30 years, but the detectors haven’t caught a single WIMP.

Even though the sensitivity of dark matter detectors has improved by more than five orders of magnitude since the early 1980s, all results so far are compatible with zero events. The searches for axions, another popular dark matter candidate, haven’t fared any better. Coming generations of dark matter experiments will cross into the regime where the neutrino background becomes comparable to the expected signal. But, as a colleague recently pointed out to me, this merely means that the experimentalists have to understand the background better.

Maybe in 100 years they’ll still sit in caves, deep underground. And wait.

Meanwhile others are running out of patience. Particle dark matter is a great explanation for all the cosmological observations that general relativity sourced by normal matter cannot explain. But maybe it isn’t right after all. The alternative to using general relativity and adding particle is to modify general relativity so that space-time curves differently in response to the matter we already know.

Already in the mid 1980s, Modehai Milgrom showed that modifying gravity has the potential to explain observations commonly attributed to particle dark matter. He proposed Modified Newtonian Dynamics – short MOND – to explain the galactic rotation curves instead of adding particle dark matter. Intriguingly, MOND, despite it having only one free parameter, fits a large number of galaxies. It doesn’t work well for galaxy clusters, but this clearly shows that many galaxies are similar in very distinct ways, ways that the concordance model (also known as LambdaCDM) hasn’t been able to account for.

In its simplest form the concordance model has sources which are collectively described as homogeneous throughout the universe – an approximation known as the cosmological principle. In this form, the concordance model doesn’t predict how galaxies rotate – it merely describes the dynamics on supergalactic scales.

To get galaxies right, physicists have to also take into account astrophysical processes within the galaxies: how stars form, which stars form, where do they form, how do they interact with the gas, how long do they live, when and how they go supernova, what magnetic fields permeate the galaxies, how the fields affect the intergalactic medium, and so on. It’s a mess, and it requires intricate numerical simulations to figure out just exactly how galaxies come to look how they look.

And so, physicists today are divided in two camps. In the larger camp are those who think that the observed galactic regularities will eventually be accounted for by the concordance model. It’s just that it’s a complicated question that needs to be answered with numerical simulations, and the current simulations aren’t good enough. In the smaller camp are those who think there’s no way these regularities will be accounted for by the concordance model, and modified gravity is the way to go.

This correlation – the mass-discrepancy-acceleration relation (MDAR) – so they emphasize, is not itself new, it’s just a new way to present previously known correlations. As they write in the paper:

“[This Figure] combines and generalizes four well-established properties of rotating galaxies: flat rotation curves in the outer parts of spiral galaxies; the “conspiracy” that spiral rotation curves show no indication of the transition from the baryon-dominated inner regions to the outer parts that are dark matter-dominated in the standard model; the Tully-Fisher relation between the outer velocity and the inner stellar mass, later generalized to the stellar plus atomic hydrogen mass; and the relation between the central surface brightness of galaxies and their inner rotation curve gradient.”

McGaugh, hero of act 1, points out that the sample size for this simulation is tiny and also pre-selected to reproduce galaxies like we observe. Hence, he thinks the results are inconclusive.

In act 3, Mordehai Milgrom, the original inventor of MOND – posts a comment on the arXiv. He also complains about the sample size of the numerical simulation and further explains that there is much more to MOND than the MDAR correlation. Numerical simulations with particle dark matter have been developed to fit observations, he writes, so it’s not surprising they now fit observations.

“The simulation in question attempt to treat very complicated, haphazard, and unknowable events and processes taking place during the formation and evolution histories of these galaxies. The crucial baryonic processes, in particular, are impossible to tackle by actual, true-to-nature, simulation. So they are represented in the simulations by various effective prescriptions, which have many controls and parameters, and which leave much freedom to adjust the outcome of these simulations [...]The exact strategies involved are practically impossible to pinpoint by an outsider, and they probably differ among simulations. But, one will not be amiss to suppose that over the years, the many available handles have been turned so as to get galaxies as close as possible to observed ones.”

This one uses a code with acronym EAGLE, for Evolution and Assembly of GaLaxies and their Environments. This code has “quite a few” parameters, as Aaron Ludlow, the paper’s first author told me, and these parameters have been optimized to reproduce realistic galaxies. In this simulation, however, the authors didn’t use this optimized parameter configuration but let several parameters (3-4) vary to produce a larger set of galaxies. These galaxies in general do not look like those we observe. Nevertheless, the researchers find that all their galaxies display the MDAR correlation, regardless.

This would indicate that the particle dark matter is enough to describe the observations.

However, even when varying some parameters, the EAGLE code still contains parameters that have been fixed previously to reproduce observations. Ludlow calls them “subgrid parameters,” meaning they quantify physics on scales smaller than what the simulation can presently resolve. One sees for example in Figure 1 of their paper (shown below) that all those galaxies have a pronounced correlation between the velocities of the outer stars (Vmax) and the luminosity (M*) already.

One shouldn’t hold this against the model. Such numerical simulations are done for the purpose of generating and understanding realistic galaxies. Runs are time-consuming and costly. From the point of view of an astrophysicist, the question just how unrealistic galaxies can get in these simulations is entirely nonsensical. And yet that’s exactly what the modified-gravity/dark matter showoff now asks for.

48 comments:

"The alternative to using general relativity and adding particle is to modify general relativity so that space-time curves differently in response to the matter we already know."

Just to be clear, depending on the definition, it is not the alternative but rather one alternative. Another alternative (though probably not relevant in the MOND debate in its simple form) is macroscopic dark matter, which you have posted about before.

" John Moffat shows that modified gravity – the general relativistic completion of MOND"

The term "modified gravity" refers to a whole range of theories, not all of which are designed with MOND in mind and which might not reproduce MOND phenomenology. Most are also not by Moffat. I guess you meant "John Moffat's version of modified gravity, which is also a relativistic completion of MOND".

"As a start, the no-dark-matter case is confirmed to work badly:the need for dark matter starts near the cluster centre, where Newton’s law is still supposed tobe valid. This leads to the conundrum discovered by Zwicky, which is likely only solvable if thetheories assume additional (dark) matter. Neutrinos with eV masses serve well without altering thesuccesses in (dwarf) galaxies."

With a slight abuse of terminology, I count macro dark matter as particle dark matter. It's stuff, basically, as opposed to a modification of general relativity.

Regarding the terminology. Well, Moffat calls this model modified gravity, I was thinking the polite thing would be to use this name. You're right though, it's unfortunate, as there's more than one way to modify gravity. Be that as it may, I am not aware of any other modified gravity model that comes even remotely close to Moffat's in fitting observations at this point. Best,

Have the MOND people any idea how they explain the Bullet Cluster and the CMB data? You can hold up any theory you want (greetings to Mr. Quine), but if it doesn't explain all evidence it is just not very attractive.

"Have the MOND people any idea how they explain the Bullet Cluster and the CMB data? You can hold up any theory you want (greetings to Mr. Quine), but if it doesn't explain all evidence it is just not very attractive."

Bullet cluster? Unclear. CMB? Very unclear. However, you could just as well use your second sentence to rule out the standard model, since there is a lot of data which it doesn't explain (hence MOND).

The difference is that MOND folks admit that the CMB is good support for the standard model.

Which is more handwaving? MOND is not a relativistic theory; maybe we can understand the CMB in a relativistic extension? All galaxy phenomenology which MOND explains with one adjustable parameter will be shown to follow unambiguously from conventional physics once computers are powerful enough to do simulations which are detailed enough?

Einstein-Cartan-Kibble-Sciama gravitation given achiral spacetime curvature is GR. Chiral spacetime torsion is indistinguishable given achiral challenges (socks on a left foot). Chiral challenges are opposite shoes on a left foot, non-identical minimum action trajectories, EP violation. The first paragraph is sourced.

IMHO most LCDM folks are stringers and SUSY advocates. CDM is their last refuge in salvaging SUSY. These folks have formidable influence in the community evidenced by winning awards without proof that their model works. Worse, they will never admit they are wrong. Yes Bee, they are prepared to dig in for at least a millenia

Thank you, Dr. Hossenfelder, for your enlightening view into the progress of MOND and related theories. I forget who said it (von Neumann?) that given few observations and many parameters a person could "fit an elephant" into whatever theory suited his/her purposes. This seems to be the case with current non-Einsteinian gravity theories and the parameters that multiply like rabbits when various scalar, vector, tensor and spinor fields are introduced into the basic formalism.

I'm surely wrong, but I can't help seeing dark matter and dark energy as a kind of Michaelson-Morley aether that doesn't exist. The cosmological constant, coupled with some kind of consistent modified gravity theory, seems much simpler, especially in view of the lack of experimental evidence to date. Again, much thanks.

As they re-materialize on the transporter deck shaking their heads at the readings in their hand-held dark matter detectors, Helbig looks over at Hossenfelder, shaking his head ... "It's stuff, Bee, but not as we know it."

There is some hope that the LISA Pathfinder (LPF) will be able to falsify at least some of the modified gravity theories deriving from MOND in a passage or passage through a gravitational saddle point (where the net Newtonian acceleration of gravity becomes small compared to the MOND acceleration parameter) during its extended mission. (Of course, if LISA Pathfinder actually sees a signal, that would strongly motivate a follow on mission to explore this.)

See https://arxiv.org/abs/1404.0313 for some more details.

The extended mission test should actually start soon, assuming the LPF Lagrange point mission is not itself extended, but it would take some months using WSB techniques to get LPF into the proper orbit for the saddle point fly-throughs.

The LPF web site has not been updated in some time, but my understanding is that this is going forward.https://www.elisascience.org/articles/lisa-pathfinder/lpf-science/testing-newtonian-gravity-earth-sun-saddle-point

"Witten, though relatively famous, is only the fourth author on the paper. The second and third are much more famous among astrophysicists." Wow, that is an objective attitude: fame as degree of importance and calling Witten "relatively famous", amuzing. Also, fourth author because we publish using alphabetical order.

Who is "we"? It certainly isn't the norm in astrophysics to publish in alphabetical order. Maybe Witten is fourth author because the order is alphabetical, OK. My comment was not a slur at Witten, but rather at the tendency to drop famous names. Why mention any of the authors at all? Or if so, mention them all, or "first author et al." as when citing?

Witten is not an astrophysicist, so it is no surprise that Tremaine and Ostriker (two of the most famous astrophysicists there are) are more famous than Witten among astrophysicists. No, fame doesn't necessary correlate with degree of importance, but I was commenting on fame (since I wondered why only Witten's name was mentioned), not on importance.

I may be missing it but I don't understand how MOND-type theories would explain observations of the Bullet Cluster. Maybe they would dispute the interpretation of the evidence but I thought it was claimed at 8σ significance that modification of gravity alone could not account for the measurements.

Michael, I think the Bullet cluster idea as THE evidence for dark matter is very much oversold idea ( for example things like 8 sigma etc). As Sabine said, you should the paper mentioned and then decide. also there are counterexamples to Bullet Cluster such as Trainwreck cluster, which again no one talks about.

The question is, is Moffat gravity a MOND-type theory? The paper is about MOG. There might be some confusion here, in that other "modified-gravity" theories might be ruled out, perhaps including MOND, but not MOG.

(One also has to be careful also when some paper has a discrepancy of a factor of 2 or 3 between theory and observations and this is touted as agreement while the same factor of 2 or 3 is considered a dismal failure for MOND.)

Moffat gravity has MOND as a limit. MOND isn't as universally applicable as MOG for the same reason Newtonian gravity isn't as universally applicable as general relativity. Hence, complaining that MOND doesn't fit this or that cluster is entirely pointless without asking first whether it even applies in that limit.

And, yes, as Shantanu says, the Bullett cluster is oversold. It's an appealingly simple explanation that goes well in the popular science media, but it's not as clear cut as it's been portrayed. My understanding is that some clusters are hard to fit both with modified gravity and LambdaCDM, and maybe that shouldn't be so surprising. What we observe today depends on the history of these systems, and exactly what all these galaxies did over the course of billion years is difficult to model.

According to Mannheim, "solar system phenomenology is left intact", as he is describing in https://arxiv.org/abs/1610.08907.

More problematic seems to be the quantum version of Confirmal Gravity (if standard quantum theory is applied) due to ghost states. But this should not really be an argument with respect to this dark matter discussion, since (at least to my knowledge) neither MOND nor MOG intend to say anything about quantum gravity. Even GR (which is the basis of LCDM) is problematic with quantization.

So, why not taking Confirmal Gravity more serious in explaining galactic (and solar system) observations? Only 4 parameters to fit well a lot of rotation curves is not a bad job.

The Event Horizon Telescope will help in discarding some MOG theories and give a strong test for GR. Any MOG theory that survives this test will not only be the right theory to solve the galaxy rotation curve problem but will certainly point the way to QG.

Thanks for the link. I see that the Moffat version of MOG is likely to fail because on their website the EHT folks report that they have observed a fuzzed version of the shadow which is already 30% less than that predicted by GR .Avi Leob http:// www.nature.com/news/good-data-are-not-enough-1.20906 would agree with you and Moffat in exploring other interpretations of astrophysical data rather than try to filter it in favor of LCDM

I haven't posted your comment because it starts with a string of several hundred capital letters (mostly Ns and Zs).

You don't speak NZ?

Not sure what you did, but you might want to repost that.

I don't know either. Could be fell asleep. I apologize for that. Also my appreciation of your being so relaxed about it.

I didn't post another one because it contains wrong statements about cosmology that I don't want to confuse other readers

That could've been a couple of different things. If you mean my comment about the correlations you raised being connected with redshift periodicity, it's just that the majority of the underlying discoveries of those correlations is the same fella that, who is also the originator of the periodicity observation.

I don't know where the line is drawn for qualifying as a 'wrong statement in cosmology'. Is that reasonable when concerning work carried out by excellent scientists/astronomers that the mainstream chooses to ignore?

If on the other hand you mean my comment about the magnitude/velocity correlation - I did get that wrong in that I misattributed what you were talking about to the original magnitude/velocity observations back circa 1970. Apologies for that as well. p.s. My comment was reasonable given that context